It now is possible to utilize stable isotopes and gas chromatography-mass spectrometry to perform new kinds of kinetic studies in newborn infants. The objectives of the current proposal are as follows: (1) To use this technology in order to measure the turnover of the secretory proteins albumin and fibronectin in premature newborn infants. This series of investigations involves the administration of the labelled amino acid (15N) glycine to these infants and the subsequent determination of the appearance of this isotope in the albumin and fibronectin. by comparing this rate of appearance with the enrichment of urinary (15N) hippuric acid, which is a marker for intra-hepatic (15N) glycine enrichment, it is possible to measure the fractional synthetic rate (FSR) (day-1) of these secretory proteins. We will ascertain whether the FSR varies with the gestational age and the diet of these infants. The second objective is: (2) To utilize stable isotopes in order to determine whether premature infants are capable of the synthesis of the amino acids glycine and cysteine. This series of studies involves the administration of 13C and 15N precursors of these compounds and the subsequent determination of isotopic abundance in the amino acids of interest. In this way it will be possible to determine in a precise fashion if these amino acids are essential for premature infants. The synthesis of cysteine and glycine will be correlated with the gestational age of the infant. Kinetic studies such as these are of importance in establishing the nutritional needs of premature infants.